1. Technical Field
The present invention relates to a method for gasification of carbonaceous material in two phases in a circulating fludized bed reactor in which solid particles contained in gases exhausted from the reactor are separated and recycled to the reactor. The invention also relates to an apparatus for carrying out the method.
It is an object of the invention to provide a method and an apparatus for producing gas of low tar content by utilizing fluidized bed technology for gasification of a carbonaceous material such as coal, brown coal or peat.
2. Prior Art
Two-staged gasification is known technology. The so called HTW gasifier (Hochtemperatur-Winkler) could be mentioned as an example. In an apparatus of this kind the gasifying agent of the second phase is, however, supplied "unselected" directly to the gasified mixture which, in addition to coal and tarry substances, contains gases (hydrogen, carbon monoxide, methane) which are the desired end products of the gasification process. Oxygen then reacts primarily with these gaseous substances although a reaction with the coal would be advantagous for the product of the gasification. Contact of the coal and the supplied aqueous steam also decreases which results in an incomplete coal conversion and low grade gas.
British patent specification No. 1506729 discloses a method of gasifying carbonaceous material in a circulating fluidized bed reactor which is devided into two parts and in which the carbonaceous material is supplied to the upper part of a gasification reactor. When the first stage of the gasification, for example pyrolyses of the volatiles, during which pyrolysis for example tarry substances build up, takes place in the upper part of the reactor, the tarry substances remain in the product gas. The amount of these tarry substances depends on the gasified material and the gasification temperature and it may, e.g. in case of coal, be small. The object of this prior method does not primarily seem to be to produce gas of low tar content either but to achieve an as good a coal conversion as possible. The primary object of the present invention, on the other hand, is to produce gas containing as little tar as possible. Thus the upper part of the reactor is used for removing tar by raising the temperature. An essential feature is that the temperature is increased by combusting coal and not gaseous components. To achieve this, a spouting type fluidized bed is used.
Finnish patent specification No. 62554 discloses a two-staged gasification method, in which the beginning of the gasification process is carried out in the upper part of the reactor, as in the method of GB No. 1506729 mentioned above, and the tarry substances remain inn the product gas.